Chromium-resistant bacterial populations from a site heavily contaminated with hexavalent chromium

Chromium-containing industrial effluents are primarily responsible for envi ronmental contamination by toxic and highly mobile, hexavalent chromium. Th e dilution plate-count method, using media amended with Cr(VI) at concentra tions ranging from 0 to 1000 mg L-1, was used to compare the sizes of Cr(VI )-resistant bacterial populations from a soil contaminated with 25 100 mg k g(-1) total Cr [12 400 mg kg(-1) Cr(VI)] to those isolated from a slightly contaminated soil (99.6 mg kg(-1) total Cr) and two other soils without any history of Cr contamination. Bacterial populations resistant to 500 mg L-1 Cr(VI) were isolated from all soils except the heavily contaminated soil. To determine whether Cr-resistant bacterial populations were indigenous to both the contaminated and the uncontaminated soils, enrichment cultures con taining Cr(VI) at concentrations ranging from 0 to 1000 mg L-1 were employe d. Bacterial populations, as high as 10(5) (colony forming units) CFU g(-1) soil, tolerant of 500 mg L-1 Cr(VI) were isolated from all soils within 48 h of enrichment suggesting that the presence of aerobic Cr(VI)-resistant b acterial populations is unrelated to contamination levels or contamination history. However, identification of these resistant bacteria using fatty ac id profiles was unsuccessful suggesting that these populations may have uni que characteristics. Fungal colonies resistant to 1000 mg L-1 Cr(VI) were r outinely isolated from both uncontaminated and contaminated soils. The resu lts suggest that Cr-resistant microorganisms may be present in soils; even those with no history of Cr contamination.